The present invention concerns an improved means intended to damp the oscillating movements of hoisting mechanisms suspended in a link the upper end of which is pivotally mounted about a first horizontal shaft on the outer end of e.g. a crane jib and at the lower end of which the hoisting mechanism is pivotally mounted about a second horizontal shaft, the vertical longitudinal medium plane of which forms an angle, preferably a right angle, to the vertical longitudinal medium plane of the first shaft.
The hoisting device may be in the form of a gripping means which is largely used in logging and other forestry operations in connection with vehicle mounted cranes for loading and unloading timber and similar work. In order that the gripper jaws, when spread apart, be able to efficiently find their way between the logs in a log pile and then grab as large a number thereof as possible, the gripper must be pivotally suspended so as to be able to move in one or several directions. Such hoisting gear, when freely suspended, will however easily start oscillating during the crane jib operations. Also when the crane jib movements have ceased, the hoisting gear continues to oscillate on account of its cardanic suspension. The oscillation or pendulum system possesses such inertia that it takes considerable time before the oscillations die away, which considerably delays the work.
Various solutions have been suggested to diminish the time of oscillating. In accordance with one suggestion telescopic shock absorbers are inserted between the crane jib and the hoisting device. However, shock absorbers are liable to damage during loading operations. In addition, they considerably limit the angular deflections or swings of the hoisting device.
In accordance with another suggested solution the cardan link joints are provided with a friction lining, e.g. in the form of rubber bushings provided at the link hub. The rubber is biased through axial compression. One disadvantage of this type of damper means is that the compression pressure must be reset from time to time. Because the damper means is arranged about the rotational shafts of the cardan link, the frictional force will act with a comparatively small leverage to the centre of rotation. The braking moment consequently will be relatively small. In addition, the angular speed of the pendulum becomes low. The damper acts close to the area of static friction, which results in considerable wear.